Heating with antiferromagnetic particles in a high frequency magnetic field



HEATING WITH ANTIFERROMAGNETIC PARTI- CLES IN A HIGH FREQUENCY MAGNETICFIELD Jerome R. White, Wilmington, Del., assignor to E. I. du

, Pout dc Nemours and Company, Wilmington, Del., a

corporation of Delaware No Drawing. Continuation-impart of applicationsSer. No. 302,489, Aug. 8, 1963, and Ser. No. 316,542, Oct. 16, 1963.This application Apr. 2, 1964, Ser. No. 356,957

20 Claims. (Cl. 229-17) ABSTRACT OF THE DISCLOSURE A method of coating,laminating, sealing cartons and the resulting product involving heatinga coating or adhesive containing multi domain antiferromagneticparticles by exposure to an alternating magnetic field of at least 10megacycles per second.

Cross references This application is a continuation-in-part ofapplications Ser. Nos. 302,489 filed Aug. 8, 1963 and 316,542 filed Oct.16, 1963 and now abandoned.

Background In many processes for heating heat-activatable materials, itis desired to have complete control of the degree of heating, theprecise location of heating and the rate and duration of the heating andcooling cycles. One such process involves heat-sealing flexiblesubstrates such as paper, paperboard, glassine, etc. having coatedthereon a heat-activatable protective adhesive organic coating, tothereby form cartons or other containers, such as milk cartons, orcartons or packages for other food products. In such a process theamount or degree of heating is important. Excessive heat will destroy ordamage the organic coating and/or char the substrate. Also the coatingmay melt throughout the entire layer and diffuse into the poroussubstrate, thus destroying the adhesive effectiveness thereof.Insufficient heating will not properly activate the coating thusprohibiting the formation of adherent bonds. The location of the heatingis important since it is only necessary to heat the coating in theprecise areas where the coating must be activated, such as in the areaof overlapping flaps. In some processes it is absolutely critical thatthe heat-activatable material not be heated except in certain preciseareas. The rate and duration of the heating and cooling cycles is alsoimportant. If the rate and duration of heating are too long, the processwill be forced to operate at an undesirably slow pace to permit adequateheating. Similarly, if the rate of cooling is too long, the individualwork pieces cannot be immediately stacked or placed adjacent to othersurfaces, thus hampering the rate of output of the process. On the otherhand, if the duration of heating is too short, the coating will not beproperly activated. Similar problems are encountered in other processesfor heating other heat-activatable materials.

Practitioners of the art have proposed several processes for heatingheat-activatable materials where such problems are encountered. However,no completely satisfactory methods have heretofore been devised. Forexample, U.S. Patent 2,393,541, issued to Kohler, describes a techniquewhereby conductive metal particles are dispersed in the heat-activatablematerial which is applied to a substrate as desired. The assembly isthen subjected to a magnetic field causing the metal particles to heat,primarily by hysteresis losses, thereby activating the heatactivatablematerial. While this technique, as described,

. United States Patent 3,391,345 Patented July 9, 1968 is suitable formany purposes, the rate of heating the metal particles is inherentlyunduly slow, requiring several minutes to properly heat theheat-activatable material. Thus, this technique is not suitable for highspeed, mass production processes.

Another technique is described in U.S. Patent 2,457,- 758, issued toVere, whereby heat-scalable surfaces may beactivated by providing anelectrically conductive band of 'rnetal particles in contact with theheat-scalable surfaces, which is then subjected to an electromagneticfield. The electrically conductive band is heated by inductive heating(eddy current losses), which activates the heatsealable surfaces. Again,this technique, as described, is suitable for many purposes. Theelectrically conductive band is heated in a fraction of a second,provided it is heavily loaded with the electrically conductive metalparticles. However, it is difficult to control the degree of heatingusing this technique, since by inductive heating, the temperature of themetal particles continues to rise above Curie point thereof. This inturn, can lead to deg. radation of the heat-activatable material,resulting in charring of the substrate, and other undesirable effects.

An object of this invention is to provide an improved process forheating heat-activatable materials. Another object is to provide animproved process for heating heatactivatable materials using analternating magnetic field. A further object is to provide an improvedprocess for heating heat-activatable materials using an alternatingmagnetic field, which provides complete control of the degree ofheating, the precise location of heating and the rate and duration ofthe heating and cooling cycles. An additional object is to provide astructure comprising a heat-aotivatable material which structure isadapted for use in such processes.

Summary These and other objects are fully attained by the presentinvention which provides the process of heating a heatactivatablematerial comprising contacting said material withfinely-dividednonconductive antiferromagnetic particles and while' in contact,subjecting said material and said particles to an alternating magneticfield of at least 10 megacycles per second.

Detailed description The terminology used to describe the magneticproperties of materials, unfortunately, is not uniform in theliterature. Antiferromagnetic materials, as contemplated in thisinvention, are defined and distinguished from other materials inWaldron, Ferrites, D. Van Nostrand Company, Ltd., London (1961), p. 31,and Van Der Ziel, Solid State Physical Electronics (1957), pp. 552553.It has been noted that these antiferromagnetic materials generally areuncompensated, as explained in the cited Van Der Ziel reference.

These antiferromagnetic materials are sulfides, oxides and mixtures ofoxides of chromium, manganese, iron, cobalt, and nickel, either alone ortogether with oxides or mixtures of oxides of the alkali metals (i.e.,lithium, sodium, potassium, and rubidium), alkaline earth metals(i.e.,;beryllium, magnesium, calcium, strontium, barium, and radium),rare earth metals (i.e., lanthanum and the other elements of atomicnumbers 57 to 71 of the periodic table), and other metals, such ascopper, zinc, vanadium, titanium, and aluminum, wherein the compound hascertain crystal structures, in particular, 2. spinel, garnet,perovskite, or pyrrhotite crystal structure. The preferredantiferromagnetic materials are the ferrites, that is, the oxides andmixtures of oxides of chromium, manganese, iron, cobalt, and nickeleither alone or in combination with the other metals, described above,wherein the compound has the s pinel crystal structure. These materialsare all familiar to those skilled in the art. Cf. Smit and Wijn,Ferrites, p. 136 (1959); Waldron, Ferrites (supra) at pp. 3035; US.Patents 2,452,529 to 2,452,- 531, issued Oct. 26, 1948 to J. L. Snoek;US Patent 2,886,529, issued May 12, 1959 to C. L. Guillaud; and Van DerZiel, Solid State Physical Electronics (supra), at p. 555.

These antiferromagnetic materials are electrically nonconductive, thatis, they have electrical resistance of at least 10- ohm-cm. Thesematerials typically have electrical resistances of up to 10 ohm-cm, andhigher.

It is critical that the antiferromagnetic material be finely divided.However, it is also essential that these particles be multidomained,that is, each particle must contain more than one, and preferably many,Bloch walls which separate regions of magnetization, and which aretermed domains. These domains are thin laminar transition regions inwhich the magnetization changes from the direction existing outside thewall on one side to the direction existing on the outside of the otherside of this wall, the directions differing by either 180 or 90 angulardegrees. Thus the lower limit on particle size is determined by thefactor that the particle must be multidomained. The precise size of thedomain varies with different materials. Particles on the order of 0.01micron in size have been knOWn to be multidomained. The particles may beas large as about microns in size. Preferably, the particles range from0.1 to 5 microns in size. Particle size is critical to this invention,primarily to insure proper heating characteristics, but also to obtainproper suspension of the particles in a liquid medium, and to render thecoating of such particles smooth to the touch.

It should be noted that these antiferromagnetic materials, inherently,are extremely friable. Therefore, ordinary grinding equipment, such as aball-mill, may be used to conveniently obtain the requisitefinely-divided particle sizes. By way of contradistinction, theconductive metal particles used heretofore are far less friable. Whenthese previously used conductive metals are reduced in size, theparticles fuse together, or smear, as they approach the size of thefinely-divided particles used in this invention. In fact, it isextremely difiicult to obtain particles of conductive metals less thanabout 75 microns in size. A few complicated methods are available forproducing such finely-divided conductive metal particles such asparticular complex chemical precipitations in liquid media, and 'byvapor deposition onto a fluid surface. However, these latter techniques,obviously, are totally unsuitable in uses such as contemplated for thepresent invention.

For convenience, these finely-divided multidomain nonconductiveantiferromagnetic particles are referred to hereinafter asantiferromagnetic particles.

The Nel temperature of the antiferromagnetic material determines themaximum temperature to which the material is heated upon subjection toan alternating magnetic field, that is, once the material reaches itsspecific Nel temperature the magnetic effects cease, and the temperatureof material will not be raised further. This effect is similar to thatknown in the art which is associated with the Curie temperature offerrromagnetic metals. However, the antiferromagnetic materialstypically have a much more abrupt transition at their Nel temperaturethan do the metals at their Curie temperature. Furthermore, theantiferromagnetic materials generally possess a relatively high anduniform permeability over the whole temperature range from roomtemperature to the Nel temperature so that considerable heating isproduced at all temperatures between the starting temperature and thedesired final temperature. This results in both extremely rapid heatingand fine temperature control.

Thus, the degree or amount of heating is precisely controlled byselection of an antiferromagnetic material having a particular Neltemperature. Such materials are commercially available having variousNel temperatures,

and therefore, selection of the proper antiferromagnetic material iswithin the ordinary skill of practitioners of the art. Normally, it isnecessary to select an antiferromagnetic material having a Neltemperature of at least the activation temperature of theheat-activatable material to be heated. The upper temperature is limitedonly by the degradation temperature of the heat-activatable materialand/or the degradation temperature of any substrate or other adjacentbodies.

The alternating magnetic field must have a frequency of at least 10megacycles per second, or preferably 40 to 2500 megacycles. Ordinaryconductive metal particles as used in prior art processes aremagnetically responsive, i.e., become heated, when subjected to analternating magnetic field in the kilocycle per second range, or onemegacycle at the most. However, the antiferromagnetic particles used inthis invention are not sufficiently responsive to such frequency,heretofore considered high frequency. Instead, they must be subjected toa field having a frequency at at least 10 megacycles per second in orderto heat at a practical rate. The particles reach their Nel temperaturewithin milliseconds upon subjection to such extremely high frequencies,whereas the ordinary conductive metal particles may require on the orderof several minutes to heat. Upon removal from, or disruption of, themagnetic field, the particles cool to room temperature, again withinmilliseconds.

It should be noted that the conductive metals used in the art heretoforein conjunction with relatively low frequency magnetic radiation, are forall practical purposes, completely inoperable in the present inventionwhich employs the extremely high frequency alternating magnetic field ofat least 10 megacycles per second and preferably, at least 40 megacyclesper second. Upon subjection to such frequencies, the conductive metalparticles spark, resulting in tracking, charring of the substrate andnonuniform heat patterns.

In passing, it is also noted that the density of the conductive metalsused in the art heretofore generally is about twice that of theantiferromagnetic materials used in this invention. Consequently, theprior art conductive metal particles are diflicult to suspend in aliquid medium to produce a satisfactory ink or the like.

To insure optimum efiiciency, the magnetic field must have a fluxdensity of at least 50 gauss, with 50 to 500 gauss being the normaloperating range, and to 300 gauss being the preferred range.

This invention provides a unique heat source for a wide variety ofheat-activatable materials. The term heat-'activatable materials referto materials which can be heated to a particular temperature to secure apractical accomplishment. For example, certain solid thermoplastics maybe melted to secure adhesive properties associated with the so-calledhot-melt adhesives. Volatile liquids may be evaporated by heating theliquids to their vapor points. Gaseous and liquid materials may beheated to the temperature at which they become reactive in particularchemical reactions. Curable resins may be heated to the temperature atwhich chemical crosslinking occurs.

Thus, this invention may be used to activate heatactivatable adhesivessuch as the various thermoplastic hot-melt adhesives, for example,polymer-modified petroleum wax compositions. Particularly preferredpolymermodified petroleum wax compositions are those containing olefinpolymers, such as homopolymers and copolymers of ethylene, propylene,isobutylene, etc., especially ethylene copolymers, that is, ethylenepolymers containing one or more additional copolymerized monomer, suchas ethylene/vinyl acetate, ethylene/ethyl acrylate,ethylene/1,4-hexadiene, ethylene/methyl methacrylate,ethylene/methacrylic acid, and the like. One preferred compositioncomprises 50 to 99.9% by weight petroleum wax, 0.1 to 50% by weight ofan ethylene/vinyl acetate copolymer having a vinyl acetate content of 15to 35% by weight and a melt index of 0.1 to 500, and 0 to 40% by weightof a resin, esterified resin, rosin or esterified rosin.

As is familiar to those skilled in the art, these heatactivatableadhesives are useful for adhering a variety of different types ofsubstrates such as paper (including the so-called paperboard orcardboard), various metals, plastics, leather, glass, etc., either tolike or ditferent substrates. For example, by use of theseheat-activatable adhesives in accordance with this invention one metalsubstrate can be adhered to another metal substrate, metal can beadhered to paper, paper to paper, leather to paper, and so forth.Consequently, in accordance with this invention there is provided anovel structure comprising a substrate having coated on at least aportion thereof a heat-activatable adhesive composition and acomposition comprising the antiferromagnetic particles describedhereinbefore.

This invention is especially useful for preparing cartons or othercontainers such as milk cartons, frozen food containers and the like,using such heat-activatable adhesives. By such a process, substratessuch as carton or container blanks are provided with a coating of theheatactivatable adhesive, at least over the areas of the surfacesthereof to be adhered together. When the aforedescribed polymer-modifiedpetroleum wax compositions are employed, the entire substrate is coated,at least on one side, to provide a protective barrier coating. Theadhesive on at least one of the surfaces must be in intimate contactwith the finely-divided multidomain antiferromagnetic particles. Thus,the particles may be in a physical admixture with the adhesive on one orboth of the surfaces to be adhered. However, it is preferred to use theunique structure provided by this invention which comprises a paper(including the so-called paperboard and cardboard), substrate havingcoated on at least a portion of the surface thereof (i.e., at least onthe areas of the surfaces to be adhered) a polymer-modified waxcomposition and a composition comprising mul-tidom'ain nonconductiveantiferromagnetic particles having particle sizes of less than 5microns.

The coating comprising the antiferromagnetic particles is generallyprepared from a dispersion of the particles and a binder such as anatural or synthetic resin or glue, preferably polyvinyl acetate, in aliquid dispersing medium or solvent for the binder such as a loweralcohol, such as methanol, ethanol, isopropanol, etc. This coatingcomposition is applied to one or both of the surfaces to be adhered inthe areas where adherence is desired. The topcoating is then applied atleast over the areas covering the above-described coating comprising theantiferromagnetic particles, and preferably is applied to at least oneentire surface of the substrate. The substrate is then folded, asdesired, and is passed through the alternating magnetic field, with thesurfaces to be adhered being in contact with each other. The particlesare heated to the Nel temperature within milliseconds, and then cooledto a nontacky temperature also within milliseconds, thus permittingextremely fast mass production.

This invention may also be used to dry ordinary printing inks for use onhigh speed printing presses, whereby finely-divided antiferromagneticparticles are dispersed in the printing ink. Immediately after thesubstrate is printed, it is passed through an alternating magneticfield, causing the particles to heat and evaporate the solvent used inthe ink, effecting virtually instantaneous drying of the ink, andpermitting the printed substrates to be immediately stacked afterprinting. Moreover, since only the ink is heated, and since it is heatedand then cooled all within a fraction of a second, the paper itself isnot detectably heated. Thus, water which is inherently present in thepaper, is not evaporated, thereby eliminating any possible shrinking ofthe paper.

Similarly, this technique can be employed with inks which contain anoxidizable liquid vehicle, such as the lithographic inks based uponlinseed oil. By use of this invention the ink vehicle can be virtuallyinstantaneously heated to the proper oxidizing temperature and thencooled, with the above-indicated advantages inherently accruing.

This invention also provides a technique for heating gases or liquids toproper temperature for reactions in chemical'processes, by passing thegases or liquids through a fixed or fluid bed of finely-dividedmultidomain antiferromagnetic particles which are continually subjectedto an alternating magnetic field. Such a process may be very effectivelyconducted where the particles are embedded in a catalyst support used inthe process.

The following example serves to further illustrate this invention: Acarton of the type generally described in U.S. Patent 2,695,745 wasprepared by coating only the flap areas to be sealed of the carton witha composition comprising finely-divided, nonconductive,antiferromagnetic particles. This composition was prepared from acommercial ferrite consisting essentially of about 10%, by weight, NiO,6% ZnO, 1% MnO and 83% Fe O sold under the registered traedmark or tradename Ceramag 11 by Stackpale Carbon Company, Electronic ComponentsDivision, St. Marys, Pa., and having a Nel temperature of 385 C., aninitially permeability of 115, and a volume resistivity of 2.5 X 10ohmcm. at 30 C. This ferrite was ball-milled for about 16 hours inWater. The mill slip was then filtered, dried and crushed to an averageparticle size of about 3 microns. The ferrite particles were then mixedwith a 30% solution of polyvinyl acetate (molecular weight of5000-20,000) in a mixture of methyl and ethyl alcohol, to obtain acomposition containing 67% ferrite and 33% polyvinyl acetate solution,having a viscosity of 500-1000 centipoise. This composition was thencoated on to the carton blank flaps as described above. The entirecarton blank was then coated with an ethylens/vinyl acetate-paraffin waxcomposition. The carton blank was folded and positioned on a mandrel,adjacent to an electrode structure in the pressure pad. A -80 megacycleper second alternating magnetic field of 125 gauss was generated. Thecoating on the flaps reached its sealing temperature withinmilliseconds, whereupon the alternating magnetic field was disrupted,and the surfaces of the adjacent flaps in contact with each other cooledto room temperature and fused together within 200 milliseconds. Noexternal cooling of the mandrel or pressure pad was required. Strong,paper-tearing, nonleaking bonds were obtained. The coating did not meltelsewhere on the carton. A durable carton was formed.

By repeating the foregoing example using a flux density of 200 gauss,the coating on flaps reached its sealing temperature within 20milliseconds, at which time the alternating magnetic field wasdisrupted, and the surfaces of the adjacent flaps in contact with eachother cooled to room temperature within 100 milliseconds. Again, strong,paper-tearing, nonleaking bonds were obtained; the coating did not meltelsewhere on the carton; and a durable carton was formed.

Although this invention has been described in considerable dctail, thoseskilled in the art will recognize many alterations and variations ofthese details which may be made Without departing from the spirit andscope of this invention. Accordingly, it will be understood that thisinvention is not intended to be limited except as defined by thefollowing claims.

I claim:

1. The process comprising heating a heat-activatable material comprisingcontacting said material with finelydivided multiclomainantiferromagnetic particles having an electrical resistance of at least10-2 ohm-cm. and while in contact, subjecting said material and saidparticles to an alternating magnetic field having a frequency of atleast 10 megacyclcs per second.

2. The process of claim 1 wherein the said particles are less than about5 microns in size.

3. The process of claim 2 wherein the said field has a flux density ofat least 50 gauss.

4. The process of claim 3 wherein the said particles are ferrites.

5. The process of heating a heat-activatable material comprisingcontacting said material with multidomain antiferromagnetic particleshaving an electrical-resistance of at least 10" ohm-cm. and particlesizes of about 0.1-5 microns and while in contact, subjecting saidmaterial and particles to an alternating magnetic field having afrequency of 40 to 2500 megacycles per second and a flux density of 100to 300 gauss.

6. The process of claim 5 wherein said particles are ferrites.

7. The process of adhering together at least two surfaces wherein atleast one of said surfaces is coated with a heat-activatable adhesive atleast on the area to be adhered, and at least one of said surfaces iscoated with a composition comprising finely-divided multidomainantiferromagnetic particles on the area to be adhered, said particleshaving an electrical resistance of at least ohm-cm. and a sufficient Neltemperature to activate said adhesive, said process comprisingcontacting said surfaces to be adhered and while in contact, subjectingsaid surfaces to an alternating magnetic field having a frequency of atleast 10 megacycles.

8. The process of claim 7 wherein the said particles are less than about5 microns in size.

9. The process of claim 8 wherein the said field has a flux density ofat least 50 gauss.

10. The process of claim 9 wherein the said particles are ferrites.

11. The process of claim 10 wherein the said heatactivatable adhesive isa polymermodified petroleum wax composition.

12. The process of adhering together at least two paper surfaces whereinat least one of said surfaces is coated with a polymer-modified waxcomposition at least on the area to be adhered, and at least one of saidsurfaces is coated on the area to be adhered with a compositioncomprising multidomain antiferromagnetic particles having an electricalresistance of at least 10 ohm-cm. and particle sizes of about 0.1-5microns, said process comprising contacting said surfaces to be adheredand while in contact, subjecting said surfaces to an alternating magnetic field having a frequency of 402500 megacycles per second and aflux density of 100 to 300 gauss.

13. The process of claim 12 wherein the said particles are ferrites.

14. The process of claim 13 wherein the said polymermodified petroleumwax composition comprises 50 to 99.9% by weight of petroleum wax, 0.1 to50% by weight of an ethylene/vinyl acetate copolymer having a vinylacetate content of 15 to 35% by Weight and a melt index of 0.1 to 500,and 0 to 40% by weight of a member of the group consisting of resins,esterified resins, resins and esterified rosins.

15. A structure adapted for heat activated adherence on exposure to analternating magnetic field at a frequency of at least 10 megacyclescomprising a substrate having coated on at least a portion of thesurface thereof a heatactivatable adhesive composition in contact withfinelydivided multidomain antiferromagnetic particles having anelectrical resistance of at least 10 ohm-cm.

16. The structure of claim 15 wherein the said particles are less thanabout 5 microns in size.

17. The structure of claim 16 wherein the said particles are ferrites.

18. A structure comprising a paper substrate having coated on at least aportion thereof a polymer-modified wax composition in contact withmultidomain antiferromagnetic particles having an electrical resistanceof at least 10- ohm-cm. and particle sizes of less than 5 microns.

19. A paper carton blank having coated on the end flaps thereof to beadhered, a composition comprising multidomain antiferromagneticparticles having an electrical resistance of at least 10" ohm-cm. andparticle sizes of less than 5 microns, and having coated on at least theside thereof which comes into contact with the end flaps coated withsaid particles, a polymer-modified petroleum wax composition.

20. The paper carton blank of claim 19 wherein the said polymer-modifiedpetroleum wax composition comprises 50 to 99.9% by weight petroleum wax,0.1 to 50% by weight of an ethylene/vinyl acetate copolymer having avinyl acetate content of 15 to 35% by weight and a melt index of 0.1 to500, and 0 to 40% by weight of a member of the group consisting ofresins, esterified resins, rosins and esterified rosins.

References Cited UNITED STATES PATENTS 2,087,480 7/1937 Pitman 219-472,280,771 4/1942 Dufour et al 11793.l 2,364,790 12/1944 Hemming 21910.412,922,865 1/1960 Schattler et a1. 21010.41 3,001,891 9/1961 Stoller117-93.2 3,181,765 5/1965 Bonzagni et al. 229--3.5 3,249,658 5/1966Hodges 26425 OTHER REFERENCES Gurevich, A. G., Ferrites at MicrowaveFrequencies, Consultants Bureau, N.Y., 1963 (Russian text published1960), pp. 1 and 19.

EARL M. BERGERT, Primary Examiner.

W. E. HOAG, Assistant Examiner.

